scanning: Add browser proxy

Add browser proxy to give Scanning UI access to ScanningHandler
functions.

Bug: 1059779
Change-Id: Iad0f72305ae315c3079393ff077c784ec9b8f101
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2497427
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821831}

chromeos/components/scanning/resources/BUILD.gn

@@ -20,6 +20,7 @@ js_type_check("closure_compile_module") {
     ":scanning_app",
     ":scanning_app_types",
     ":scanning_app_util",
+    ":scanning_browser_proxy",
     ":select_behavior",
     ":source_select",
   ]
@@ -96,6 +97,7 @@ js_library("scanning_app") {
     ":scanner_select",
     ":scanning_app_types",
     ":scanning_app_util",
+    ":scanning_browser_proxy",
     ":source_select",
     "//third_party/polymer/v3_0/components-chromium/polymer:polymer_bundled",
     "//ui/webui/resources/js:i18n_behavior.m",
@@ -121,6 +123,10 @@ js_library("source_select") {
   ]
 }
 
+js_library("scanning_browser_proxy") {
+  deps = [ "//ui/webui/resources/js:cr" ]
+}
+
 js_library("mojo_interface_provider") {
   deps = [
     "//chromeos/components/scanning/mojom:mojom_js_library_for_compile",

chromeos/components/scanning/resources/scanning_app_resources.grd

@@ -43,6 +43,7 @@
     </includes>
 
     <structures>
+      <structure name="IDR_SCANNING_APP_BROWSER_PROXY_JS" file="scanning_browser_proxy.js" type="chrome_html" />
       <structure name="IDR_SCANNING_APP_MOJO_INTERFACE_PROVIDER_JS" file="mojo_interface_provider.js" type="chrome_html" />
       <structure name="IDR_SCANNING_APP_SELECT_BEHAVIOR_JS" file="select_behavior.js" type="chrome_html" />
       <structure name="IDR_SCANNING_APP_TYPES_JS" file="scanning_app_types.js" type="chrome_html" />

chromeos/components/scanning/resources/scanning_browser_proxy.js

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/**
+ * @fileoverview A helper object used by the Scanning App UI in chromeos/ to
+ * provide access to the ScanningHandler which invokes functions that only exist
+ * in chrome/.
+ */
+
+import {addSingletonGetter, sendWithPromise} from 'chrome://resources/js/cr.m.js';
+
+/** @interface */
+class ScanningBrowserProxy {
+  /** Initialize ScanningHandler. */
+  initialize() {}
+
+  /**
+   * Requests the user to choose the directory to save scans.
+   * @return {!Promise<string>}
+   */
+  requestScanToLocation() {}
+}
+
+/** @implements {ScanningBrowserProxy} */
+export class ScanningBrowserProxyImpl {
+  /** @override */
+  initialize() {
+    chrome.send('initialize');
+  }
+
+  /** @override */
+  requestScanToLocation() {
+    return sendWithPromise('requestScanToLocation');
+  }
+}
+
+// The singleton instance_ can be replaced with a test version of this wrapper
+// during testing.
+addSingletonGetter(ScanningBrowserProxyImpl);